The invention relates to a piezoelectric injector for fuel-injection systems of internal combustion engines.
In piezoelectric injectors, the thermal expansion of housing parts of the piezoactuator roughly corresponds to the working stroke of the piezoactuator. On the other hand the thermal expansion of piezomaterials is approximately zero. Therefore, it is known that hydraulic compensating members or housing combinations made of materials with different thermal expansion values are used.
From MTZ Motortechnische Zeitschrift 56 (1995) 3, pages 142-148, FIG. 8, a piezoelectric injector with a rod-shaped piezoactuator is known which closes an injection nozzle drilled in a seat bore in the expanded, i.e. charged state. The high pressure that prevails at this point is metered by the nozzle needle when the actuator discharges. The tensioning of the system can be adjusted by cup springs. The pretensioning force must be high enough for the nozzle to seal off the full fuel pressure with the actuator charged.
The low thermal expansion of the piezoceramic is partially compensated by a combination of CFCs (chlorofluorocarbons) and metal in the injector housing.
In piezoelectric injectors of this type, the direction of movement of the piezoactuator when subjected to flow is opposite that of conventional solenoid valves.
A goal of the invention is to improve the piezoelectric injector of the type referred to above specifically with regard to a simpler design in which the valve housing can consist of materials such as steel or aluminum without the thermal expansions of these materials exerting an unfavorable influence on the accuracy of the valve strokes.
This goal is achieved by providing a piezoelectric wherein the actuating part comprises a pressure pin that passes through the piezoactuator, with a head part resting on the piezoactuator, wherein the pressure pin and piezoactuator are of approximately the same length and are made of similar characteristic thermal expansion ceramic material or ceramic-like material, and wherein sealing piece can be lifted off its valve seat by piezoactuator which is extended when in the charged state.
The special arrangement and location of the actuating part in the piezoelectric injector as well as the motion reversal of the piezoactuator when subjected to flow permits a simple design which, despite the additional use of materials such as steel or aluminum conventionally employed in valve housings, causes no adverse effects on function as far as exact valve strokes are concerned.
The lengthwise expansions that unavoidably occur under thermal loads thus do not produce any negative effect on the unimpeded closing function of the valve.
In addition, if there is an electrical defect in the injector, there is no leakage at the nozzle, which can lead to engine damage, especially in high-pressure systems using the common rail principle, because the nozzles no longer close, said nozzles opening or closing depending on the position of the sealing piece cooperating with a control piston on the back of the nozzle needle in the piezoelectric injector.
The pressure pin and piezoactuator that have the same length and are made of the same ceramic material can also consist of a ceramic material, Invar for example.